Synthesis of combinatorial compound libraries on bead supports is a well-established method for generating chemical diversity for screening against targets of pharmacological relevance. Such libraries may be synthesized as bulk populations or discrete sublibraries with or without identifier tags for deconvolution. In most cases each bead carries a single unique compound and is present in a mixture of beads containing other compounds. To test the beads for activity against a pharmacological target, it is desirable to separate the beads so that each bead may be individually assayed against one or more targets. The beads carrying active compounds can be identified and the structures of the compounds elucidated. It is also desirable to separate the beads into arrays that are geometrically compatible with robotic screening systems, for example 8.times.12 arrays or other formats such as 384 or 896-well configurations.
Current methods of bead arraying include manual picking and hydrodynamic sorting in which beads are allowed to flow though an aperture and once detected are deposited into assay wells. Manual picking is slow and tedious. Hydrodynamic methods heretofore in use have been slow and the equipment is prone to clogging. Buoyancy variations within the bead library are also known to cause problems in hydrodynamic sorting. A problem common to the prior methods is that, because the beads are extremely small, typically 300 {character pullout}m(0.3 mm) or smaller, there is a tendency for two or more beads, carrying different compounds, to be deposited occasionally at a single location in the array. Still another problem encountered in bead arraying is that the beads tend to be fragile, and can be broken up into fragments especially when mechanically agitated.
These problems have recently been addressed in a bead picking apparatus by which beads are picked individually from a suspension of beads in a liquid. The suspension can be established by releasing gas bubbles from orifices at the bottom of a vessel to keep the liquid in motion. Alternatively, two immiscible liquids can be used, one having a density greater than that of the beads and the other having a density lower than that of the beads. The beads are suspended at the interface of the two liquids.
The bead picking apparatus comprises a set of hollow needles. Liquid is drawn from the suspension into the internal passages of the needles through openings at an ends of the internal passages, each opening being of a size smaller than any one of the beads and shaped so that it can be substantially closed off by a bead. When a bead closes off the opening, a pressure differential is established such that the external pressure exerted on the bead closing off the opening is greater than the pressure within the passage, and the bead is thereby held in engagement with the opening. The set of needles is then withdrawn from the vessel with the beads in engagement with the openings, and the beads can then be released and deposited into wells at a remote location.
The procedure just described has been used successfully, but has certain drawbacks. It is difficult to use gas bubbles to prevent settling of beads. Elaborate measures must be taken in order for gas bubbles to keep beads in suspension. For example, the vessel for containing the suspension of beads may be constructed with separate compartments, one for each needle in the picking apparatus. The compartments have sloping walls, and each compartment has its own gas-releasing orifice. This arrangement does not lend itself readily to use with a bead picker in which the needles are very close to one another. Therefore, in the case in which beads were to be deposited in a conventional well plate having eight columns of wells, it was necessary to use a picker comprising four needles spaced from one another so that they were alignable with every other well in a row of eight wells, and to index the picker laterally in order to deposit beads in odd-numbered wells of a row in a first step, and thereafter deposit beads in the even-numbered wells of the same row.
In the case of immiscible liquids it is difficult to achieve a satisfactory spatial distribution of beads at the liquid-liquid interface so that beads readily become attached to every needle of the bead picker.
Another difficulty encountered with the prior bead picking apparatus is that excess beads adhering to the needles are jarred loose by stopping the picker suddenly, or by bringing the needles into contact with a stop. Using this method, the excess beads not readily dislodged without a significant risk of dislodging the desired beads attached to the openings of the needles.
Still another difficulty with the prior bead picking apparatus is the clogging that can occur especially in the case in which a needle picks up a broken bead.